This invention relates generally to vehicle steering devices, and more particularly, this invention relates to vehicle steering devices having variable ratio steering.
Motor vehicles, such as cars and trucks, require a steering system to control the direction of travel of the vehicle. The steering system controls the direction of travel of the vehicle through the control of one or more sets of road wheels. Such steering systems commonly transmit a driver's intent from a steering wheel to the road wheels via a mechanical steering linkage. Thus, movement of the steering wheel by the driver causes a corresponding movement of the road wheels. Hydraulic and/or electric motor assisting systems are commonly used in combination with such mechanical systems. These assisting systems reduce the driver effort necessary to actuate the mechanical system.
For a vehicular steering system with active steering, such as that used in an automotive active front steering system, a given motion of the hand-wheel to an input shaft may be supplemented by an additional motion, such as that from a differential steering actuator, including, for example, an augmentation motor, to an output shaft, translating into a motion of the steerable road wheels that does not necessarily correspond to the given motion of the hand-wheel. Consequently, when the differential steering actuator is inactive, the motion of the steerable road wheels directly corresponds to the hand-wheel motion due to the articulated mechanical linkage, just as in conventional systems.
The term “active steering” relates to a vehicular control system, which generates an output that is added to or subtracted from the front steering angle, wherein the output is typically responsive to the yaw and/or lateral acceleration of the vehicle. Active front control steering may improve vehicle handling stability on a variety of road conditions. Stability control may be continuously active. For higher vehicle speeds, vehicle sensitivity of steering may be smaller. At lower vehicle speeds, park solution sensitivity may be increased and driver workload reduced. Thus, in some situations, an active steering control system may react more quickly and accurately than an average driver to correct transient handling instabilities. In addition, active steering can also provide for variable steering ratios in order to reduce driver fatigue while improving the feel and responsiveness of the vehicle. For example, at very low speeds, such as that which might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented using an active steering system in order to provide an increased steering angle to the steerable road wheels.
Active Front Steering “AFS” typically uses a differential mechanism to achieve a controlled ratio change or position augmentation. These mechanisms possess characteristics that make inclusion in an automotive steering system difficult. These characteristics include lash from input to output, friction to ground, and the speed change through the differential device.
U.S. Pat. No. 6,199,654 shows a vehicle steering apparatus that has an electric motor connected with a steering shaft and thus the electric motor assembly rotates together with the steering wheel. The electric motor (stator) is rotated and has a spiral cable for electric power for any angle of the steering shaft in the connected electric motor. Since all of the embodiments of the apparatus include a stator of the electric motor rotating together with the steering shaft, a spiral cable is implemented to maintain electrical continuity for proper functioning of this apparatus. Rotation of a complete electric motor has large inertial forces and has additional assembly of a spiral cable that has additional issues including noise, friction, inertia, and durability.